In an important fundamental advance, molecule hunters have added a small but long-sought trophy to their collection of ephemeral compounds. For the first time, chemists have managed to capture and study the elusive singlet carbene hydroxymethylene (Nature 2008, 453, 906).
The hunt for hydroxymethylene, or HCOH, has been something of a puzzling pursuit. Theoretical studies suggested the molecule should be stable enough to isolate, but until now all attempts to catch the carbene failed.
To flush out the wily carbene, researchers led by Peter R. Schreiner of Germany's Justus Liebig University and Wesley D. Allen of the University of Georgia first heated glyoxylic acid in a high vacuum. Under these conditions, glyoxylic acid eliminates CO2 to generate HCOH, which the chemists then trapped in a matrix of solid argon at 11 K.
Hydroxymethylene doesn't stay caught for long, though. The carbene rearranges to formaldehyde with a half-life of two hours. This maneuver surprised the researchers because at 11 K there's not enough thermal energy for HCOH to overcome the high rearrangement reaction barrier. Rather, the team determined, the hydroxyl hydrogen bypasses the barrier via quantum tunneling.
In a commentary that accompanies the report, Markku Räsänen of Finland's University of Helsinki notes that the findings "might cast fresh light on the chemistry of these fascinating compounds, and could even lead to the discovery of new reactions."